It is known that organic gases from compositions based on carbon can be used to form hermetic coatings on optical fibers made, for example, of silica. European Patent Application EP-A-308 143 describes an optical fiber that is coated with a carbon hermetic coating as it passes through a reactor in which the fiber is put in contact with a reactive gaseous medium. That apparatus essentially comprises a cylindrical reactor placed vertically, and the fiber moves down through the reactor. A reactive gaseous medium is fed in at the bottom of the reactor, and escapes therefrom at the top thereof. Air locks are formed at the ends of the reactor to prevent any interaction between the atmosphere and the reactive gaseous medium.
It has been observed that deposits of carbon soot build up rapidly at the top and, above all, at the bottom of such an apparatus. The presence of soot in the reactor makes it possible for the soot to settle on the fiber and to degrade the properties of the already-formed hermetic coating. Therefore, the maximum length of fiber that can be obtained before the apparatus needs cleaning is only a few kilometers.
Since the cost of manufacturing optical fibers depends directly on the length of fiber that can be manufactured in a single run by being treated in such an apparatus, attempts have been made to reduce soot buildup.
Document EP-A-518 318 describes a method and apparatus for manufacturing an optical fiber having a hermetic coating. In that method and apparatus, soot is prevented from building up in the reactor by forming a continuous film of liquid on the inside wall of the reactor; as the liquid runs off, it entrains with it any soot that forms in the reactor. It is mentioned that that method and apparatus make it possible to obtain fiber in continuous lengths longer than 100 km. Unfortunately, that method is complex and difficult to implement, and forming a continuous film of liquid along the walls poses problems. The reactor flares going downwards, and it includes devices for purifying the liquid after use and the spent gases. In that reactor, the reactive gaseous medium flows at a constant speed in the same direction as the fiber, and then its speed decreases at an end-piece whose end corresponds to the reactor flare.
Document EP-A-571 915 describes other another apparatus for producing an optical fiber having a hermetic coating and that can be very long since soot build-up in the reactor is reduced. According to that document, the speed of advance of the reactive gaseous medium is reduced gradually along the fiber, in the portion in which chemical vapor deposition is performed. That document describes, in particular a comparison performed between a conventional reactor and a reactor in which the crosssectional area increases gradually, that reactor making it possible to manufacture a fiber that is twice as long as the fiber that can be manufactured with the conventional reactor.
The above-mentioned problem of soot building up in the reactor is due to the fact that a portion of the reactive gaseous medium decomposes, thereby forming soot which is deposited on the wall of the reactor. In the above-mentioned document EP-A-518 318, the result is obtained by the soot being mechanically entrained as it forms, whereas, in above-mentioned document EP-A-571 915, the result is obtained by reducing the speed of the gas stream so that the soot is kept away from the walls of the reactor, with the speed of advance being reduced.